Metallic parts and components such as those used in aircraft engines sometimes are too complex to manufacture using standard techniques such as casting or machining due to part complexity because the components are manufactured from different materials. These parts also often require repair after damaged areas are removed. In order to accomplish the manufacture and/or repair of such parts and components, brazing is accomplished. Brazing entails utilization of a filler material that has melting point that is below the melting point of the base metals that must be joined or that is under repair. After application of the filler material, the region under repair is heated to an elevated temperature above the melting point of the filler metal but below the melting point of the base metal or metals. On cooling, the filler metal provides the joint between the parts or provides the repair area. If desired, suitable heat treatments can be applied to the region under repair to achieve diffusion between the base metal and the added filler metal so that the area that either forms the joint or that has been repaired can have properties, such as melting point, that are closer to the properties of the base metal than the original braze metal that was used as a filler.
Because of the high temperatures experience by aircraft engine parts, it is not always feasible to heat just the area that requires brazing. It frequently is necessary to prepare an article for brazing and than braze the entire article in a furnace under a protective atmosphere or under a vacuum. Because of the size of the vacuum furnaces and the furnaces having protective atmospheres, it is not economically feasible to manufacture or repair a single part or component. Instead, the parts or components are processed in batches of 40-50 parts by loading them into the furnace and achieving the desired atmosphere. For example, brazing using vacuum furnace processing for a batch of parts requires a time of about four hours to pump down the furnace to the required vacuum, ramping the part to the required temperature, holding the parts at the required brazing temperature for the required amount of time and cool down. These furnaces require a substantial amount of time to achieve the necessary vacuum, heat up slowly and cool down slowly, all due to their large volume and mass. This extensive time does not include the time required to load and unload the batch of parts. And of course, this batch processing is limited to parts and components comprised of the same base materials and the same braze filler materials, and frequently to parts of the same or of similar configuration.
Another technique that is sometimes used to accomplish brazing is induction brazing. This process has been used to process individual parts in protective atmospheres, but is very limited. This process requires the design of a unique induction coil for each part configuration. The induction coil must closely conform to the part configuration in order to achieve uniform heating of the part or workpiece. Because of this drawback, the part configuration, and hence the induction coil, is generally limited to simple geometries typically having uniform thicknesses or displaying simple symmetry.
The current state of the art utilizes three stage vacuum furnaces to permit small batch flow, while still providing uniform heating of the components or parts being brazed. However, these vacuum furnaces are still large and display little improvement in cycle time, typically requiring three-hour cycle times. In addition, the vacuum furnace equipment is expensive and complex, so that replacement and repair of the equipment can become a major consideration.
What is needed is apparatus that permits the brazing of individual parts in a vacuum or under a protective atmosphere, in a quick, efficient and economical manner without restrictions on component or part geometry. As an option, the equipment should allow the individual parts to be processed on as continuous basis as well as on a batch basis. Ideally, the equipment should not be complex and expensive, and when possible, should not be large, so that it can occupy a small area of a manufacturing facility such as a table top, or if desired, can be mounted on a lab bench.
Apparatus is provided for controlled-atmosphere brazing of parts or components having cross-sectional dimensions of up to twenty-four inches. The apparatus comprises a susceptor having a physical boundary, typically at least one wall, that separates an interior of the susceptor from its exterior. The wall has an internal diameter that is slightly larger than the maximum cross-sectional dimension of the parts or components that are to be brazed, so that the parts or components can be received within the inner diameter or interior of the susceptor. There is virtually no limitation on the length of the part or component that can be brazed by the present invention. A means for heating the susceptor is provided. For example, the outer surface of the susceptor is surrounded by a heating source that can apply heat substantially uniformly to the susceptor. The heating source is capable of heating the susceptor sufficiently so that the interior of the susceptor can reach a temperature sufficient to braze the components inside the susceptor. The susceptor containing the parts to be brazed and heating source are placed within a chamber that can provide a desired protective environment, such as a vacuum, an inert gas atmosphere, a reducing gas or nitrogen.
The heating source is coupled to a temperature controller which controls the output of the heating source in the conventional manner to assure that sufficient energy is provided by the heating source to maintain the temperature on the interior of the susceptor within predetermined limits. The temperature of the space within the inner diameter of the susceptor may reach a temperature as high as 2400xc2x0 F. However, the temperature within the susceptor must be controlled so that a minimum temperature above the liquidus of the braze filler material is reached, but that a maximum temperature set below the melting point of the base material of the parts or components being brazed is not exceeded. Temperatures can be controlled within xc2x11xc2x0 by many controllers currently available, and temperature ranges within xc2x15.5xc2x0 F. at 2200xc2x0 F. are required to be maintained by the controller. The temperature at various points within the susceptor should vary by no more than xc2x115xc2x0 F. and preferably by no more than about xc2x110xc2x0 F., well within the capability of the controllers. The actual processing temperature selected will depend upon the base material being brazed and the braze filler material being used. The temperature controls are an important factor, as the ability to control the temperature within the susceptor is dependent on the ability of to control the heating elements.
After the parts have been placed into the chamber and the desired atmosphere has been achieved, heat is applied by the heat source to the susceptor until a predetermined temperature has been reached within the susceptor. The susceptor acts to redistribute and radiate heat through its interior, so that a substantially uniform temperature is attained throughout the interior of the susceptor. This uniform temperature within the interior of the susceptor also means that the parts or components that are located within the interior of the susceptor also attain a uniform temperature, even if there are substantial differences in cross sectional configuration along the length of the part or component.
The characteristics of the susceptor are that is must be stable at the elevated temperatures at which brazing is performed, so that it does not interfere with the brazing. The susceptor must be able to radiate heat across its boundary, and ideally should be a good conductor of heat so that hot spots do not form on the susceptor. Such hot spots could adversely affect the temperature uniformity along the interior of the susceptor. The susceptor should be manufactured using standard manufacturing techniques, and if desired, should be capable of being manufactured into complex shapes, although for most applications, simple configurations having constant cross-section are adequate. The susceptor ideally should be oxidation-resistant and should be resistant to thermal fatigue and thermal stress shocks.
An advantage of the present invention is that the susceptor when heated uniformly by a heat source can provide a uniform temperature distribution throughout its interior. Thus, parts or components that are positioned within the susceptor are heated in a uniform manner, even when cross-sections and thicknesses are substantially dissimilar and otherwise are heated unevenly. Thus, the problems associated with temperature uniformity and convection currents as well as location of the part or component in proximity to the heat source such as currently exists within state of the art furnaces are eliminated.
Another advantage of the present invention is that an induction coil can be used as a heat source for the susceptor, which can be loaded into a small chamber slightly larger than the susceptor, if desired after it has been loaded with the parts or components. Because parts of varying sizes and cross-sectional configurations can be loaded into the susceptor, simultaneously if desired, only one induction coil that can be arranged over the susceptor is required, even though a variety of parts or components can fit within the susceptor.
Still another advantage of using the susceptor of the present invention is that the use of a small furnace for processing of small batches of parts or components or for individual parts or components allows for drastic reduction of processing times. The desired atmospheres can be achieved in small furnaces much more quickly than in large furnaces, an heat up and cool down can be accomplished more expeditiously.
Still another advantage of the present invention is that the interior of the susceptor can be closed at either end to substantially isolate convection currents that may be present in the furnace from the interior of the susceptor. In addition, if the ends of the susceptor are sealed, it is possible to provide attachment points for vacuums or for protective gases so that the desired atmosphere can be obtained in the interior of the susceptor, without the necessity of maintaining the desired atmosphere throughout the entire furnace interior.
Yet another advantage of the present invention is that the brazing operations can be done using small furnaces that are positioned on work benches and occupy about {fraction (1/10)} of the space that current furnaces occupy. Because many small furnaces can occupy the same amount or less space than current large furnaces, it is possible to braze many parts of different configuration and of different materials and at different temperatures simultaneously in the different furnaces. Each individual part can be processed more quickly than processing can be accomplished using present techniques
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.